Ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. interconnection between buried conductive levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic structures are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a thermoplastic polymeric binder, plasticizers and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, punching, screening, stacking and laminating, the greensheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
A major problem in the production of multilayer ceramic structures is the occurrence of gapping between the metal via and the ceramic material which occurs as a result of the volume shrinkage mismatch between the metal vias and the bulk ceramic, and also in part by the insufficient adhesion between the metal via and the surrounding ceramic.
A typical via paste composition may comprise metal particles, an ethyl cellulose organic binder, solvents, plasticizers and flow control agents.
It is known that the addition of small amounts of powdered glass or ceramic particles (frit) to a via composition will reduce the stress resulting from the thermal expansion mismatches between metals and ceramics. The amount of frit is kept small to maintain the highest possible electrical conductivity required for high performance circuitry. Useful paste compositions contain about 10% of the powdered frit as part of the powdered metal component.
However, even after addition of a small amount of frit to, for example, a copper paste, the volume shrinkage of the copper paste is greater than 40%, even after the highest possible solids loading. In fact, the theoretical volume shrinkage of a copper paste containing 88 wt % copper is 55%. The typical volume shrinkage of ceramics on sintering, on the other hand, is approximately 40%. This difference in the total volume shrinkage between the conductor and ceramic (i.e., the volume shrinkage mismatch) results in the formation of gaps between the vias and the bulk ceramic.
To date, it has not been possible to maintain via integrity with regard to conductivity, while matching shrinkage rates and attempting to promote bonding of the metal to the dielectric.
For example, U.S. Pat. No. 4,880,684 (Boss et al.) describes advantages of using glass-ceramic composites, including low dielectric constant, favorable coefficient of thermal expansion, and the ability to sinter at comparatively low temperatures (850.degree. C.-1,000.degree. C.), making possible their use with metals such as gold, silver and copper. A uniform gap is created between the ceramic and the associated internal metal in order to provide an expansion zone between the thermally mismatched materials. Boss et al. teach that the hermeticity disadvantages associated with the gap can be overcome by the use of capture pads, which provide hermeticity, relieve stress and provide a planar base for mounting devices. Boss et al. thus teach accommodation, rather than elimination, of the gap resulting from volume shrinkage mismatch between metal filled via or paste and ceramics.
U.S. Pat. No. 4,687,597 (Siuta) describes the use of copper conductor compositions in microcircuits, and the problems of oxidation of copper during firing. Siuta teaches a fritless copper conductor composition suitable for overprinting on copper, consisting of finely divided copper particles, coarse copper particles, a reducible heavy metal oxide, and optionally a refractory metal and/or high surface area noble metal. In the case of tungsten as a refractory metal, the tungsten acts as a reducing agent and serves as an oxygen scavenger.
U.S. Pat. No. 4,493,789 (Ueyama et al.) discloses mixing aluminum oxide, silicon dioxide, or similar oxide with a high melting point metal powder (such as molybdenum or platinum) in a paste for ceramic metallization. The metal particle size is from 0.3 to 8.0 microns and the weight percent of oxide powder added to the metal powder is from 0.1 to 3.0. There is no mention of controlling shrinkage rate.
U.S. Pat. No. 4,409,261 (Kuo) discloses the use of a copper powder and glass frit mixed into a paste for use in copper metal process. The copper powder has a particle size from 1.0 to 5.0 microns while the glass frit has submicron particle size. The glass frit is any low melting point frit such as lead borosilicate glass. The copper may or may not be pure. This process eliminates the need for a protective atmosphere during firing of the metal conductor. There is no mention of controlling the shrinkage rate of copper.
U.S. Pat. No. 4,619,836 (Prabhu et al.) discloses the use of a thick film metallic ink process which uses an oxygen or equivalent plasma to remove the organic binder. It is indicated that this does not oxidize the metal particles and in fact removes any surface oxidation present. Because the ink has a small particle glass frit added to it, the composition is then fired at a higher temperature, above the glass transition temperature, to complete the formation of the metal pattern.
U.S. Pat. No. 4,540,604 (Siuta) discloses the use of small copper particles, with an average size of from 2.0 to 4.0 microns with an oxide coating, to prepare a copper metallization paste. The oxide coating decomposes upon firing and the resulting metal retains its physical characteristics even after multiple firings because of a non-cuprous metal additive in amounts of from 0.2% to 5.0% by weight.